Space junk is going to be a problem for Vera Rubin

The Vera Rubin Observatory (VRO) is different to other large telescopes, and that difference makes it more vulnerable to space junk. Other telescopes, like the Giant Magellan Telescope and the European Extremely Large Telescope, focus on distant objects. But the VRO’s job is to repeatedly image the entire available night sky for 10 years, spotting transients and variable objects.

All that space junk can look like transient events, impairing the VRO’s vision and polluting its results.

In a new research note awaiting publication, Harvard physicist/astronomer Avi Loeb points out how space junk will affect the VRO’s work. The paper, “Flares from Space Debris in LSST Images,” is available on the pre-print server arXiv. LSST is the Legacy Survey of Space and Time, the VRO’s primary observing effort.

The problem stems from space junk and also the VRO’s extreme sensitivity, a critical part of its success.

“Owing to the exceptional sensitivity of the Vera C. Rubin Observatory, we predict that its upcoming LSST images will be contaminated by numerous flares from centimeter-scale space debris in Low Earth Orbits (LEO),” Loeb writes. “Millisecond-duration flares from these LEO objects are expected to produce detectable image streaks of a few arcseconds with AB magnitudes brighter than 14.”

Our space junk problem is getting worse, as everyone knows. The ESA says that as of December 6th, 2023, there are 130 million objects in the size range of 0.1–1 cm orbiting Earth. There are also one million objects between 1–10 cm and 36,500 objects larger than 10 cm. With so many launches, the problem is getting worse. Space is a burgeoning economy, and a certain amount of junk goes with it.

Not all of those objects are in the critical Low-Earth Orbit region, but a large subset of them are. According to Loeb, this population of debris has implications for the VRO. “In this Note, we examine the implications of this LEO debris for the upcoming Legacy Survey of Space & Time (LSST) of the Vera C. Rubin Observatory in Chile,” Loeb writes.

When it comes to the VRO’s images, it’s not really the size of the debris that matters. An object’s albedo is the real problem. Albedo can scale with size, but not always.

There’s no way to measure the individual albedos of pieces of space junk, but in this work, Loeb calculates albedo by combining an object’s radius and distance with one of its sides illuminated by the sun. That yields the fraction of light that it will reflect.

We already know how space junk can reflect light because we can see it with the Zwicky Transient Facility. It’s similar to the VRO in that it detects transient light sources. “Data from the Zwicky Transient Facility (ZTF) shows that the sunlight glints from known LEO satellites generate flashes of duration 10^-3±0.5 s.” That’s an extremely brief flash.

But the VRO and its LSST will visit each patch of the sky for 30 seconds and take back-to-back 15-second exposures. The problem is that debris is moving, and rather than just a flash, it creates a streak. “The light from the flares is therefore expected to spread across no more than a few arcseconds, independently of the LSST exposure time which is 4 orders of magnitude longer,” Loeb writes.

What does that mean for the VRO?

It’s not good. According to Loeb, the number of objects that can create problematic streaks “exceeds by an order of magnitude” the number of large satellites orbiting Earth. U.S.’s Space Surveillance Network regularly tracks satellites and has built a catalogue of orbiting objects that could help the VRO manage the problem. But as Loeb points out, “Out of the entire debris population, only 3.515 × 10⁴ (351,500) objects are regularly tracked and catalogued by Space Surveillance Networks.”

Streaks of light in images are only part of the problem. There’s the more generalized problem of the combined light from all satellites and debris.

Other researchers have examined the problem and its effects on ground-based astronomy. A March 2023 paper in Nature Astronomy showed that by 2030, reflected light from space junk and functioning satellites will increase the diffuse background brightness for the VRO by 7.5%. That means the VRO’s LSST will be 7.5% less efficient. That’ll add over $20 million US to the cost of the 10-year-long LSST.

Satellites and their predictable orbits mean they should be easier to deal with. In fact, the LSST team has a plan to deal with satellites. They propose an updated scheduler that can mitigate the problem. “Overall, sacrificing 10% of LSST observing time to avoid satellites reduces the fraction of LSST visits with streaks by a factor of 2,” write the authors of a paper in The Astrophysical Journal Letters.

But junk is far more abundant. Without a solution, will LSST images be littered with noisy streaks?

It seems irrational to download the responsibility for space debris to the people trying to see the sky through it. Any long-term solution has to include two things: the cleaning up of Low Earth Orbit and an international agreement to stop polluting it even further.

The ESA is coming to terms with the space debris problem. “130 million pieces of space debris larger than a millimeter orbit Earth, threatening satellites now and in the future,” the ESA wrote when announcing their Zero Debris Charter.

“Once a week, a satellite or rocket body reenters uncontrolled through our atmosphere. Behaviors in space have to change.” While the Charter is primarily aimed at reducing the risk of collisions, it will benefit ground-based astronomy.

NASA is seeking solutions, too. Their “Detect, Track, and Remediate: The Challenge of Small Space Debris” competition is reaching out to people around the globe for innovative solutions to the problem.

Those are great initiatives, but the VRO is scheduled to see its first light in early January 2025. A solution to the problem of satellites and satellite constellations in space is likely within reach. But debris is a much thornier problem.

“However, the above numbers suggest that image contamination by untracked space debris might pose a bigger challenge,” Loeb concludes.